The technical portion of the work carried out by an optician consists in mounting a pair of ophthalmic lenses in or on the frame selected by the user, in such a manner that each lens is suitably positioned facing the corresponding eye of the user so as to perform as well as possible the optical function for which it is designed. In order to do this, it is necessary to perform a certain number of operations.
After the frame has been selected, the optician must begin by situating the position of the pupil of each eye in the frame of reference of the frame. The optician thus determines mainly two parameters that are associated with the morphology of the user, namely the pupillary distance and the height of the pupil relative to the frame.
For the frame itself, it is necessary to identify its shape, and this is generally done by means of a pattern or an apparatus specially designed to read the inner contour of the rim (i.e. the part of the frame that goes round the lens), or else from an electronic file that is prerecorded or supplied by the manufacturer.
From the above geometrical input data, it is necessary to cut each lens to shape. A lens is cut to shape for mounting in or on the frame selected by the future user by modifying the outline of the lens so as to match it to that of the frame and/or to the shape desired for the lens. Cutting to shape comprises an edging operation for shaping the periphery of the lens, and depending on whether the frame is of the rimmed type or of the rimless type with local clamping through fastener holes formed in the lens, appropriately beveling and/or drilling the lens. Edging (or cutting out proper) consists in eliminating the superfluous peripheral portion of the ophthalmic lens in question so as to reduce its outline, which is usually initially circular, to the arbitrary outline of the rim or surround of the eyeglasses frame in question, or merely to the pleasing shape desired when the frame is of the rimless type. This edging operation is usually followed by a chamfering operation which consists in dulling or chamfering the two sharp edges around the edged lens. When the frame is of the rimmed type, the chamfering is accompanied by beveling which consists in forming a rib usually referred to as a bevel, generally of triangular cross-section with a top that is rounded or interrupted by a counter-bevel on the edge face of the ophthalmic lens. The bevel is for engaging in a corresponding groove, also known as a bezel, formed in the rim or the surround of the eyeglasses frame in which the lens is to be mounted. When the frame is of the rimless type, the cutting out of the lens and optionally the dulling of its sharp edges (chamfering) are followed by appropriately drilling the lenses to enable it to be secured to the temples and to the bridge of the rimless frame. Finally, when the mounting is of the type having a rim of Nylon string, the chamfering is accompanied by grooving consisting in forming a groove in the edge face of the lens, the groove being for receiving the mounting Nylon string for pressing the lens against the rigid portion of the frame.
Usually, these edging, chamfering, and beveling operations are performed in succession on a single machine tool, known as an “edger” and provided with a set of suitable cutter/edger bits. Drilling can be performed on the edger which is then fitted with corresponding drill bits, or else on a separate drilling machine.
The optician must also perform a certain number of measurement and/or identification operations on the lens itself prior to cutting out in order to identify certain characteristics such as, for example: the optical center if the lens is a single-vision lens or the mounting cross if the lens is a progressive lens, or the direction of the axis of progression and the position of the centering point if the lens is a progressive lens. In practice, the optician marks certain characteristic points using a marker tip on the ophthalmic lens itself. These marks are used for securing a chuck receiver or centering-and-drive pad on the lens enabling the ophthalmic lens to be positioned correctly in the edger that is to give it the desired outline, corresponding to the shape of the selected frame. The pad is usually stuck temporarily on the lens by means of a double-sided adhesive. This operation is commonly referred to as “centering” the lens, or by extension, “blocking” the lens, insofar as the pad can be used subsequently for blocking purposes, i.e. for preventing the lens from moving relative to the means for cutting it to shape in a geometrical configuration that is known because of the pad.
Various devices are known for measuring and centering purposes, that operate either automatically, or manually, in order to measure various characteristics, and in particular the centering, of a single-vision or progressive ophthalmic lens before or after it is mounted in a frame. In particular, one such device is known from document FR-A 825 466, which is equivalent to document US-2003/0015649 published on Jan. 23, 2003. That measurement device includes a support arranged to receive such a lens, and on one side of the lens support, lighting means including an optical system for generating a light beam that is directed towards the ophthalmic lens situated on said support, and on a second side of the support, means for analyzing the image transmitted by said ophthalmic lens installed on said support and illuminated by the lighting means. In order to perform an analysis of the lens, the analysis being of the overall or localized mapping type, the device includes light beam separation means comprising a first beam separator mask forming a Hartmann matrix or the like and placed on the first side of the lens support in order to be illuminated, upstream from the lens, i.e. beside its convex front face, by said light beam.
A drawback of that device lies in the fact that since the Hartmann matrix is situated beside the front face of the lens, i.e. in a so-called “inverted” Hartmann configuration, the measurements taken can be insufficiently accurate, in particular when measuring optical powers of the lens or centering and/or axis characteristics.